This invention relates to recovery of polyester from waste material, and is more particularly concerned with inhibiting formation of diethylene glycol during glycolysis of ethylene terephthalate polymers to bis-(2-hydroxyethyl)terephthalate and low molecular weight polyesters thereof.
Production of polyester filaments involves formation of bis-(2-hydroxyethyl)terephthalate, condensation polymerization of this monomer to poly(ethylene terephthalate), melt-spinning into filaments, and winding into packages. The formation of waste occurs during start-up or interruption of various operations, and also results from rejection of non-standard packages. Recovery of polyester waste is an ecological and economic necessity. The problem is complicated by the great variety of products manufactured, which differ in denier and degree of polymerization. There are also differences in the polyester compositions, which include minor amounts of other materials to modify the properties. The polyester may be copolymerized with a minor amount of another compound to provide improvements in crimping, dyeability, flame retardant or anti-static properties of products.
Recovery of polyester waste has been accomplished by glycolysis of the polyester with excess ethylene glycol at elevated temperatures to form bis-(2-hydroxyethyl)terephthalate, and low molecular weight polymers thereof, for recycling in the process indicated above. A continuing problem in the recovery process has been the formation of glycol ethers, principally diethylene glycol. These copolymerize with the bis-(2-hydroxyethyl)terephthalate to form polyester having a reduced melting point and lead to filaments having reduced bulk in filling products, reduced wrinkle resistance and dye lightfastness in textile products, and to variable dyeability in textile products. Other disadvantages are noted in Japanese Patent Publication 48-61446/1973 published Aug. 28, 1973, which discloses the use of tetra-alkyl ammonium hydroxides in the depolymerization process to suppress side reactions.
Barkey U.S. Pat. No. 3,830,759 dated Aug. 20, 1974, discloses that the formation of diethylene glycol is inhibited by the use of lithium acetate dihydrate in combination with zinc acetate dihydrate and/or antimony trioxide. Example 2 illustrates the formation of prepolymer of 0.43 inherent viscosity from dimethyl terephthalate and ethylene glycol. The use of lithium acetate dihydrate resulted in prepolymer having a melting point of 260.degree.-261.degree. C and containing 1.5 mole percent of diethylene glycol. Example 3 illustrates that when recycled ethylene glycol containing 2-4 percent water was used, instead of dry ethylene glycol, the prepolymer had a melting point of 258.degree.-259.degree. C (indicating a higher diethylene glycol content than in Example 1). Example 5 illustrates glycolysis of poly(ethylene terephthalate) scrap to lower molecular weight and subsequent polymerization to form prepolymer. The use of lithium acetate dihydrate in both the depolymerization and repolymerization resulted in prepolymer having a melting point of 253.degree. C and containing 5 mole percent diethylene glycol. The patent discloses at column 7, lines 27-40, that similar experiments in which sodium acetate was substituted for the lithium acetate showed that the results obtained with sodium acetate were not as good.